One of the technological limitations of space exploration results from a spacecraft's inability to aim devices with extremely high accuracy. Current requirements for the aiming accuracy of 20 to 40-foot-long truss structures are approximately 20 nanoradian. Since the spacecraft has a number of vibration sources with undetermined frequencies (solar pressure, micrometeorites, reaction wheels, etc.), the truss structures must have some vibration damping devices. Conventional damping designs of structures of great length have proven ineffective at inhibiting vibrations at low frequencies, down to 10 Hz. These types of vibrations are very apparent in a space environment, and extremely damaging for control mechanisms.
Active aiming mechanisms can, and do, compensate for some of the vibration perturbations which a spacecraft experiences. Boris Lurie, "Balanced Bridge Feedback Pointing Control," Proc. of ACC, Atlanta, 1988, is an example of such an active feedback control system. However, for control systems to be robust, the vibration and resonances at low frequencies down to 10 Hz must be suppressed.
Conventionally, there have been a number of ways to incorporate damping into strut design. Examples of these techniques are shown in Gun-Shing Chen and Ben K. Wada, "Passive Damping for Space Truss Structures," Apr. 1988, and James F. Wilson and L. Porter Davis, "Viscous Damped Space Structures for Reduced Jitter," 58th Shock and Vibration Symposium.
These conventional designs place a viscoelastic or viscous damper in parallel with a load-carrying aluminum cylinder. These designs, however, experience several difficulties. They tend to add weight to a space structure, which excessive weight increases liftoff cost and ability. The designs are unproven at low vibration frequencies and low displacements. Finally, the level of absorption decreases with increased strut lengths, which can be a significant factor in designing large scale space structures.